1. Field of the Invention
The present invention relates to an oil ring of a type having an expander between side rails and a method of designing the oil ring.
2. Description of Related Art
In order to prevent excessive oil consumption by scraping excess oil from a cylinder of a reciprocal internal combustion engines by oil rings, it has been known, as an oil consumption control technique, to control three primary factors. These factors are the thickness of an oil film on the cylinder wall, the springiness of an oil ring, which relates to the fact that the oil ring follows up deformation of the cylinder wall, and the adhesion of an oil ring in the ring groove. In order to maintain a desired oil film thickness, a surface pressure on the cylinder wall is set to approximately 8-9 kgf/cm.sup.2. The control of the springiness of an oil ring is typically attempted by giving the oil ring a springiness coefficient greater than 0.4 so as to reduce a gap between the oil ring and the cylinder wall, caused due to deformation of the cylinder wall, so that it is as small as possible. In this instance, the term "springiness coefficient" used herein shall mean and refer to the degree of following up deformation of the cylinder wall. The degree of adhesion of an oil ring is controlled by increasing side rail stiffness so that it is as large as possible. This restrains swelling of the side rails caused by reciprocal motion of the piston which leads to oil leakage.
Typically, the particular parameters, i.e. the oil film thickness, the cylinder wall surface pressure and the side rail stiffness, are established so that these three primary factors of excessive oil consumption fall within a permissible extent obtained empirically. The parameters are further examined and verified in various combinations thereof so as to determine dimensions of the oil ring with the least excessive oil consumption. This design technique is, therefore, a trial-and-error method.
In the prior art design technique, an attempt to reduce tension of the oil ring has been made by focusing upon reducing frictional wear on or damage to the ring. Nevertheless, it has proved to be necessary to have the oil ring impart a high surface pressure on the cylinder wall as well as have low tension or be given a large springiness coefficient as well as have low tension. In any case, however, the side rails lose their stiffnesses, resulting in an increase in excessive oil consumption. Further, because of a high dependence upon experience and experiments, the design of oil rings varies widely.